Glossary · Testing & air leaks

Pascal (Pa) and "50 Pa"

Also called: pascals · 50 Pa · 50 pascals · inches of water column · inch of water · in. w.c.

A small unit of air pressure; airtightness tests hold a house at 50 pascals, about 1/2,000 of normal air pressure, so results can be compared.

What it means

A pascal is a very small amount of pressure, so building pressures are read with a sensitive gauge called a manometer. Testing every house at the same 50 Pa difference makes leakage results (CFM50 and ACH50) comparable from home to home. Duct leakage tests use 25 Pa, and combustion safety checks look at pressure changes of just a few pascals.

Why it matters in an audit

Pressure readings show the auditor how hard air is being pushed through leaks and whether exhaust fans could pull combustion gases back down a chimney (see depressurization).

Typical numbers

Reference values with their sources. Your energy advisor’s report gives the figure for your own house.

What	Value	Source
50 Pa in US units	About 0.20 inch of water column (1 in. w.c. at 60°F = 248.84 Pa)	nist.gov ↗
Duct leakage test pressure	25 Pa (0.10 in. w.c.)	basc.pnnl.gov ↗
For scale	Standard air pressure is 101,325 Pa, so 50 Pa is about 1/2,000 of it	nist.gov ↗

Sources

Learn more here

Related terms

Blower door test A test that uses a powerful, calibrated fan in an outside doorway to pull air out of the house and measure how leaky it is.
CFM50 The raw air-leakage result of a house's fan test: how many cubic feet of air per minute the fan must move to hold the house at test pressure.
ACH50 An airtightness score: how many times an hour the house's whole volume of air leaks out while a test fan pulls on it; lower means tighter.
Duct leakage test A test that pressurizes your heating and cooling ducts with a calibrated fan to measure how much air escapes through leaks.
Depressurization When more air is being pushed out of a house than is coming in, so the inside sits at slightly lower pressure than outdoors.
Worst-case depressurization test A combustion safety check run with exhaust fans on and doors set to create the lowest pressure the furnace room is likely to see.
Manometer A precise pressure gauge auditors use to read small pressure differences, in pascals, during blower door, duct leakage and combustion safety tests.

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